The progress in miniaturization and multi-layering of semiconductor devices has increased current density, which has resulted in serious electromigration (EM). Multilayer wiring techniques using copper wires, which have a high EM resistance, is thus essential for higher integration of semiconductor devices.
In a multilayer wiring technique for semiconductor devices, to improve the productivity and reliability of metal wiring, various underlayers having various types of functions are normally arranged between an insulative layer and the metal wiring. As such an underlayer, for example, a barrier layer for preventing the diffusion of metal atoms, an adhesion layer for adhering the metal wiring and the insulative layer, and a seed layer for enhancing film growth of a wiring material are known.
When using aluminum or tungsten as the wiring material, metal material such as tantalum, titanium, tantalum nitride, and titanium nitride are used as the material of the underlayer. In a multilayer wiring technique using copper wires, these metal materials are not effective for use as the material for the underlayer. This is because when a low-permittivity film is used as the insulative layer, the application of these materials to the underlayer would diffuse the moisture and oxygen contained in the insulative layer and easily oxidizes the metal contained in the underlayer. This increases the resistance of the underlayer, weakens the adhesion effect, and seriously lowers the reliability of the semiconductor device.
To resolve these problems, various types of materials for the underlayer that is applicable to a multilayer wiring technique using copper wires have been presented. Patent document 1 and patent document 2 disclose the use of a ruthenium (Ru) film or a multilayer film including an Ru film as the underlayer for copper wires. Since ruthenium oxide is conductive, the use of the Ru film prevents the resistance from increasing in the underlayer and suppresses oxidation of the copper wiring. Further, by superimposing the adhesion layer on the Ru film, the adhesion between the Ru film and the copper wires may be further improved.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-129745    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-328526